There’s nothing more frustrating than good actors trapped in a mediocre movie. Every word they say transforms the script, and just by being themselves they manage to create a character complex and engaging enough that we’re pulled in despite ourselves.

At the same time, we know the movie will never let them reach their potential, and as much as we love the actor we’re almost angry at them. If they weren’t there, we could walk away completely.

Mark Ruffalo causes exactly that kind of quandary in “Begin Again,” the latest indie movie trying to replicate the success of “Once” and failing miserably (this time, horrifically enough, by the exact same writer/director). The film squanders every opportunity to be either the fantastic satire or sweet romance it sets itself up to be, but Ruffalo is passionate, relatable and charismatic during every moment on screen. He and Keira Knightley are fantastic together, painting a subtle, moving portrait of two people putting each other back together, and between them they were nearly enough to make me forgive the movie’s every cliché.

Then some ridiculous plot contrivance would crash into me, and I would be forcibly reminded that I was stuck watching a mediocre movie.

The plot follows the basic “nobody to success” model, seemingly replicated in nearly every movie about musicians ever made. Ruffalo is a disgraced record executive, pushed out of the record label he co-founded because of his idealism and raging alcoholism. Ruffalo gives him both a deep-seated anger and flashes of vulnerability, both of which are vital in making the character believable but still likeable.

Keira Knightley is a British singer-songwriter who followed her suddenly-famous boyfriend to the U.S., only to have him almost immediately leave her for someone else. Knightley gives her character a toughness and delight in life that keeps the character interesting, though the script helps her as little as it does Ruffalo.

From there, the script could have gone three ways. It could have turned their situation into a satire of the mass-produced nature of the music industry, and both Ruffalo and Knightley have enough playfulness in their performances that they could have pulled this off beautifully.

It could have been a tender romance, the direction it chooses to take for much of the second half, and the chemistry between the two was delightful enough to overcome the script’s numerous clichés or any qualms about the age gap. Or it could have simply explored their friendship, highlighting the ways that the two had helped each other grow. That same chemistry could have translated into something beautifully illuminating there as well.

Instead, the plot toggles between the three ideas in a terminal case of indecision, seemingly switching tones and goals at the exact moment to completely destroy whatever momentum they’d just built up. The rest of the cast is lost in their smaller, even more flatly written roles, bringing what nuances to the moment they can but shackled by the enormity of the confused nonsense going on around them.

In the end, what the audience is left with is thwarted beauty, choked-off ambition, and a wish for the movie that might have been.